I keep missing 90kg and most of the time it's over the back of my head. When I play the video back, I notice my front leg looks a bit slow and the dip a bit shallow. Thoughts? I know I can get this! Thanks.

Your hips are not retreating behind you. It actually has nothing to with the short dip. Your hips are moving forward when you push down to get under the bar. You need to work on getting your hips to move straight down, or even slightly back as you go under the bar.